Somatic cell surface receptors for free C1q have been described on a variety of cells including peripheral blood mononuclear cells, fibroblasts, and endothelial cells. The presence of C1q receptors on human blood platelets has also been suggested based on the inhibitory effect of C1q on collagen-induced platelet aggregation. The aim of the present proposal is to characterize platelet C1q binding sites with regard to structure, cell surface distribution, and biologic function using highly purified C1q and two monoclonal antibodies (II1/D1 and II1/B5) which block C1q receptor functions of lymphocytes, compete with C1q for receptors on platelets, and selectively inhibit collagen-induced platelet aggregation. The distribution of C1q receptors on blood platelets will be investigated by radioligand binding assays using 125I-labeled C1q, II1/D1, and II1/B5, and fluorescence activated cell sorter analysis. The biologic function of platelet C1q receptors will be explored by studying the effect of C1q or monoclonal antibodies on platelet activation, aggregation, and release in response to stimulation by a variety of agonists including ADP, epinephrine, arachidonate, thrombin, ristocetin, and collagen. The physiologic significance of free circulating C1q and its modulation of platelet aggregation will be evaluated in patients with rheumatoid arthritis and primary biliary cirrhosis in whom increased levels of free C1q have been documented. Platelet C1q receptors will be isolated using monoclonal antibody- and C1q-affinity columns. The structural properties of these receptors will be assessed by SDS-poly-acrylamide gel electrophoresis, two dimensional isoelectric focusing, and sucrose and cesium chloride density ultracentrifugation, and compared to those of peripheral blood mononuclear cells. C1q receptors will be further characterized using 1) thiol specific fluorescent agents to detect the presence of reactive sulfhydryl groups, 2) affinity cross-linking agents, and 3) enzymatic digestion in the presence and absence of bound C1q, II1/D1, and II1/B5 to detect conformational changes induced by ligand binding. Results from these studies will provide new insights into the structural and functional relationships between platelet C1q receptors and platelet function in hemostasis.